# 2. Tutorial¶

The `mpoints`

package implements the class of state-dependent Hawkes
processes. Its key features include both simulation and estimation
(statistical inference). It also contains a module with specialised plot
tools.

## 2.1. State-dependent Hawkes processes¶

State-dependent Hawkes processes model the arrival in time of random events and their interaction with the state of a system.

A state-dependent Hawkes process consists of an increasing sequence of random times \((T_n)\), a sequence of random event types \((E_n)\) in \(\mathcal{E}\) (the set of possible event types) and a piecewise constant càdlàg state process \((X(t))\) in \(\mathcal{X}\) (the set of possible states). Here, we assume that the event space \(\mathcal{E}\) and state space \(\mathcal{X}\) are finite, with respective sizes \(d_e\) and \(d_x\).

### 2.1.1. Dynamics of events¶

Denote by \(N_e(t)\) the number of events of type \(e\in\mathcal{E}\) that have occured up to time \(t\) and by \((T^e_n)\) the arrival times of events of type \(e\). The arrival rate of events of type \(e\), denoted by \(\lambda_e\) (its mathematical name is the intensity), is of the form

The non-negative kernel \(k_{e'e}\) determines how events of type \(e'\) precipitate events of type \(e\) (by increasing their arrival rate). Notice that it depends on the state process. The kernels allow to introduce self- and cross-excitation effects. When all the kernels are null, the events behave like in a standard Poisson process with non-negative arrival rates \((\nu_{e})_{e\in\mathcal{E}}\).

### 2.1.2. Dynamics of the state process¶

The state process can only jump at the event times \((T_n)\). It jumps following transition probabilities that depend on both the current state and the event type. More precisely, denoting the history of \(N:=(N_e)_{e\in\mathcal{E}}\) and \(X\) just before time \(T_n\) by \(\mathcal{F}^{N, X}_{T_n-}\), it statisfies

where \(\phi := (\phi_e)_{e\in\mathcal{E}}\) is a family of transition probability matrices. In other words, \(\phi_e(x',x)\) is the probability of transitioning from state \(x'\) to state \(x\) when an event of type \(e\) occurs.

Notice that the state process \((X(t))\) is fully determined by its values \((X_n)\) at the event times \((T_n)\), that is, \(X_n := X({T_n})\).

Notice also that the counting process \(N\) and the state process \(X\) are fully coupled, in the sense that there is a two-way interaction: the self- and cross-excitation of \(N\) depend on \(X\) and the dynamics of \(X\) depend on \(N\).

### 2.1.3. Exponential kernels¶

At the moment, the package implements only the case of exponential kernels, that is, we only consider parametric kernels of the form

where the base rates \(\nu\) and impact coefficients \(\alpha\) are non-negative and the decay coefficients \(\beta\) are positive.

## 2.2. Setting up the model¶

From the `mpoints`

package, we import the class `HybridHawkesExp`

and the `plot_tools`

module.

```
In [1]:
```

```
import os
import numpy as np
from mpoints.hybrid_hawkes_exp import HybridHawkesExp
from mpoints import plot_tools
import seaborn # for good-looking plots
from IPython.display import set_matplotlib_formats # set the figures format to svg
set_matplotlib_formats('svg')
%matplotlib inline
```

```
/Users/maximemorariu/anaconda/envs/py36/lib/python3.6/site-packages/statsmodels/compat/pandas.py:56: FutureWarning: The pandas.core.datetools module is deprecated and will be removed in a future version. Please use the pandas.tseries module instead.
from pandas.core import datetools
```

### 2.2.1. Set the meta-parameters¶

We set the dimension of the event space \(\mathcal{E}\) and state space \(\mathcal{X}\). We also name their elements.

```
In [2]:
```

```
n_events = 2 # number of event types, $d_e$
n_states = 2 # number of possible states, $d_x$
events_labels = ['A', 'B'] # names of the event types
states_labels = ['1', '2'] # names of the states
```

We initialise an instance of a state-dependent Hawkes process with exponential kernels.

```
In [3]:
```

```
model = HybridHawkesExp(n_events, n_states, events_labels, states_labels)
```

### 2.2.2. Set the model parameters¶

We now need to input the parameters \(\phi\), \(\nu\), \(\alpha\) and \(\beta\).

```
In [4]:
```

```
# The transition probabilities $\phi$
phis = np.zeros((n_states, n_events, n_states))
phis[0, 0, 0] = 0.7
phis[0, 0, 1] = 0.3
phis[1, 0, 0] = 0.6
phis[1, 0, 1] = 0.4 # $\phi_0(1, 1)$, probability of transitioning from state 1 to 1 when an event of type 0 occurs
phis[0, 1, 0] = 0.2
phis[0, 1, 1] = 0.8
phis[1, 1, 0] = 0.4
phis[1, 1, 1] = 0.6
# The base rates $\nu$
nus = np.ones(n_events)
# The impact coefficients $\alpha$
alphas = np.zeros((n_events, n_states, n_events))
alphas[0, 0, 0] = 2
alphas[0, 0, 1] = 1
alphas[1, 0, 0] = 1
alphas[1, 0, 1] = 4
alphas[0, 1, 0] = 2
alphas[0, 1, 1] = 10
alphas[1, 1, 0] = 5
alphas[1, 1, 1] = 1
# The decay coefficients $\beta$
betas = np.zeros((n_events, n_states, n_events))
betas[0, 0, 0] = 4
betas[0, 0, 1] = 12
betas[1, 0, 0] = 14
betas[1, 0, 1] = 10
betas[0, 1, 0] = 14
betas[0, 1, 1] = 15
betas[1, 1, 0] = 10
betas[1, 1, 1] = 14
```

Set the transition probabilities \(\phi\).

```
In [5]:
```

```
model.set_transition_probabilities(phis)
```

Set the parameters that govern the dynamics of the arrival rates, that is, \(\nu\), \(\alpha\) and \(\beta\).

```
In [6]:
```

```
model.set_hawkes_parameters(nus, alphas, betas)
```

### 2.2.3. Print the model parameters¶

The model paramaters can be conveniently converted to strings via the following methods.

```
In [7]:
```

```
# Sequence of transition probability matrices, one per event type
print(model.transition_matrix_to_string(phis))
```

```
[[ 0.7 0.3]
[ 0.6 0.4]]
[[ 0.2 0.8]
[ 0.4 0.6]]
```

```
In [8]:
```

```
# Sequence of impact coefficient matrices, one per possible state
print(model.impact_coefficients_to_string(alphas))
```

```
[[ 2. 1.]
[ 1. 4.]]
[[ 2. 10.]
[ 5. 1.]]
```

```
In [9]:
```

```
# Sequence of decay coefficient matrices, one per possible state
print(model.decay_coefficients_to_string(betas))
```

```
[[ 4. 12.]
[ 14. 10.]]
[[ 14. 15.]
[ 10. 14.]]
```

## 2.3. Simulation¶

The model can now be simulated from time \(t=0\) to time \(t=7200\) (for two hours) as follows.

```
In [10]:
```

```
time_start = 0
time_end = 7200
times, events, states = model.simulate(time_start, time_end)
```

Let’s see how many events have occured in two hours.

```
In [11]:
```

```
print('Number of events: ' + "{:,}".format(len(times)))
```

```
Number of events: 37,188
```

### 2.3.1. Plot a sample path¶

The sample path from time \(t_1\) to time \(t_2\) can be plotted as follows. In the upper subplot, we show the state process \((X(t))\). The dots indicate the arrival times \((T_n)\) while their colour informs us on the event types \((E_n)\). The lower subplot displays the arrival rates \(\lambda_e\) (intensities).

```
In [12]:
```

```
t_1 = 0
t_2 = 5
seaborn.set() # set the default seaborn style for figures
fig, fig_array = plot_tools.sample_path(times, events, states, model, t_1, t_2)
```

### 2.3.2. Distribution of events and states¶

The joint distribution of \((E_n, X_n)\) can be computed and plotted as follows.

```
In [13]:
```

```
distribution = model.proportion_of_events_and_states(events, states, n_events, n_states)
fig = plot_tools.discrete_distribution(distribution, v_labels = events_labels, h_labels = states_labels,
figsize=(2, 2))
```

## 2.4. Statistical inference¶

Let’s now imagine that `times`

, `events`

and `states`

is some data
that we want to analayse. We would like to fit a state-dependent Hawkes
process to this sample path. Here, we should hopefully retrieve the
original parameters `phis`

, `nus`

, `alphas`

and `betas`

that
were used to generate this data.

Given this sample path, the model parameters can be estimated via maximum likelihood. It can be proven that the transition probabilities \(\phi\) and the Hawkes parameters \(\nu\), \(\alpha\) and \(\beta\) can be estimated independently, in spite of the strong coupling between \(N\) and \(X\).

### 2.4.1. Estimate the transition probabilities¶

The transition probabilities \(\phi\) are estimated as follows. Note that one can show that the maximum likelihood estimator is in fact given by the empirical transition probabilities.

```
In [14]:
```

```
phis_hat = model.estimate_transition_probabilities(events, states)
```

The transition probabilities can be plotted as a heatmap. The estimated transition probabilities coincide indeed with the true ones.

```
In [15]:
```

```
fig, fig_array = plot_tools.transition_probabilities(phis_hat, events_labels=events_labels, states_labels=states_labels,
figsize=(6, 3), left=0.12, right=0.88, bottom=0.18, top=0.82)
```

### 2.4.2. Estimate the Hawkes parameters¶

The package comes with a built-in method that searches for the
parameters \(\nu\), \(\alpha\) and \(\beta\) that maximise
the likelihood of events. It is based on the `scipy.optimize.minimize`

module and, by default, employs a conjugate gradient method (other
methods from `scipy.optimize.minimize`

can be called, see
documentation). More advanced users can apply their own optimisation
algorithm by calling directly the methods that compute the
log-likelihood or partial log-likelihoods and the gradient or partial
gradients (again, see the documentation).

```
In [16]:
```

```
opt_result, initial_guess, initial_guess_kind = model.estimate_hawkes_parameters(times, events, states,
time_start, time_end)
```

The method returns a `scipy.optimize.OptimizeResult`

instance which
contains the maximum likelihood estimate as a 1D numpy array. One can go
from this 1D array to the usual parameter format as follows.

```
In [17]:
```

```
# the maximum likelihood estimate in a 1D array
mle_estimate = opt_result.x
# tranform it to the usual format
nus_hat, alphas_hat, betas_hat = model.array_to_parameters(mle_estimate, n_events, n_states)
```

We check that our estimate of the base rates \(\nu\) is close to the
original value `nus`

.

```
In [18]:
```

```
print(nus)
print(nus_hat)
```

```
[ 1. 1.]
[ 0.98175138 0.98502245]
```

The kernels can be visualised by plotting the cumulative excitation functions

which provide a convenient visualisation of the magnitude of the self- and cross-excitation effects and the effective timescales at which they occur. One can interpret \(||k_{e'e}(\cdot,x)||_{1,t}\) as the average number of events of type \(e\) that are directly triggered by an event of type \(e'\) within \(t\) seconds of its occurrence, under state \(x\). We plot these functions for the estimated and true kernels.

```
In [19]:
```

```
# Estimated kernels in full line
fig, fig_array = plot_tools.kernels_exp(alphas_hat, betas_hat, events_labels=events_labels, states_labels=states_labels,
log_timescale=True, figsize=(7, 4))
# True kernels in dashed line
fig, fig_array = plot_tools.kernels_exp(alphas, betas,
tmax=1, log_timescale=True, fig=fig, fig_array=fig_array, ls='--',
left=0.12, right=0.88, bottom=0.15, top=0.85, ymax=0.7)
# Add x and y labels
txt = fig.text(0.5, 0.02, 'Time (seconds)', ha='center', fontsize=16)
txt = fig.text(0.01, 0.5, r'$||\hat{k}||_{1, t}$', va='center', rotation='vertical', fontsize=16)
```

For example, the top right subplot shows the kernels \(k_{AB}(\cdot, x)\) for the two different states \(x=1\) and \(x=2\). We were indeed able to approximately retrieve the true paramaters from the sample path, thanks to the maximum likelihood princinple.

### 2.4.3. Goodness-of-fit¶

Define the event residuals \(r^e_n\) by

where \(t^e_n\) is the time when the \(n\) th event of type \(e\) occurred (that is, \(t^e_n\) is the realisation of random variable \(T^e_n\)).

Let’s compute the sequences of residuals (one per event type) under the estimated model. We first update the model paramaeters with the estimated ones.

```
In [20]:
```

```
model.set_transition_probabilities(phis_hat)
model.set_hawkes_parameters(nus_hat, alphas_hat, betas_hat)
```

The event residuals can then be computed as follows.

```
In [21]:
```

```
residuals = model.compute_events_residuals(times, events, states, time_start)
```

Under the assumption that the sample path was generated by this estimated model, the event residuals \((r^e_n)_{n\in\mathbb{N}, e\in\mathcal{E}}\) are the realisation of i.i.d standard unit rate exponential random variables. Hence, the goodness-of-fit of our estimated model can be assessed by analysing the distribution of the event residuals.

For comparison, we also compute the event residuals under a naive Poisson model.

```
In [22]:
```

```
n_states_poisson = 1 # no state process under this naive model
model_poisson = HybridHawkesExp(n_events, n_states_poisson, events_labels, ['no state'])
# the transition probabilities are then trivial
model_poisson.set_transition_probabilities(np.ones((n_states_poisson, n_events, n_states_poisson)))
# compute the mean arrival rate and use it as the base rate
nus_poisson = np.zeros(n_events)
for e in range(n_events):
nus_poisson[e] = (events == e).sum() / (time_end - time_start)
# impact coefficients set to null to get a Poisson process
alphas_poisson = np.zeros((n_events, n_states_poisson, n_events))
# deceay coefficents, their value does not matter here
betas_poisson = np.ones((n_events, n_states_poisson, n_events))
# set the parameters that govern the arrival rates of events (intensities)
model_poisson.set_hawkes_parameters(nus_poisson, alphas_poisson, betas_poisson)
# compute the event residuals under this naive model
states_poisson = np.zeros(len(times), dtype='int') # we create a fake trivial state process
residuals_poisson = model_poisson.compute_events_residuals(times, events, states_poisson, time_start)
```

For every event type \(e\in\mathcal{E}\), we compare the distribution of \((r^e_n)\) to the standard exponential distirbution via a qq-plot. We do this under both the state-dependent-Hawkes-proces model and the Poisson model.

```
In [23]:
```

```
model_labels = ['Hawkes', 'Poisson']
fig, fig_array = plot_tools.qq_plot([residuals, residuals_poisson], shape=(1, n_events), labels=events_labels,
model_labels=model_labels,
figsize=(8, 4), left=0.085, right=0.915, bottom=0.15, top=0.85)
```

We also plot the correlogram of the event residuals time series \((r_n) := (r^1_n, \ldots, r^{d_e}_n)\) as a test of the mutual independence.

```
In [24]:
```

```
fig, fig_array = plot_tools.correlogram([residuals, residuals_poisson], labels=events_labels,
model_labels=model_labels,
figsize=(6, 5), left=0.12, right=0.88, bottom=0.12, top=0.88, n_lags=10)
```

As expected, the state-dependent Hawkes process provides an (almost) perfect fit but the Poisson model does not.

A similar study can be performed with the total residuals \(r^{ex}_n\), defined by

where \(t^{ex}_n\) is the time when the \(n\) th event of type \(e\) after which the state is \(x\) occurred.

```
In [25]:
```

```
residuals_total = model.compute_total_residuals(times, events, states, time_start)
product_labels = model.generate_product_labels()
fig, fig_array = plot_tools.qq_plot(residuals_total, shape=(n_events, n_states), labels=product_labels,
figsize=(8, 4), left=0.085, right=0.915, bottom=0.15, top=0.85, log=False)
```

```
In [26]:
```

```
fig, fig_array = plot_tools.correlogram(residuals_total, labels=product_labels, n_lags=10,
left=0.1, right=0.9, bottom=0.12, top=0.88)
```

## 2.5. Application to high-frequency financial data¶

State-dependent Hawkes processes were applied to high-frequency financial data in the following paper.

Morariu-Patrichi, M. and Pakkanen, M. S. (2018). State-dependent Hawkes processes and their application to limit order book modelling. Preprint, available at https://arxiv.org/abs/1809.08060.